(1) Field of the Invention
The present invention relates to an electrically heatable honeycomb body and a honeycomb unit comprising the honeycomb body. More particularly, the present invention relates to a honeycomb body favorably used as a honeycomb heater for, for example, purification of the exhaust gas emitted during the cold start of automobile; as well as to a honeycomb unit comprising the honeycomb body.
(2) Description of the Related Art
In recent years, metallic honeycomb structures have drawn attention as a catalyst, a catalyst carrier or the like used for purification of the nitrogen oxides (NOx), carbon monoxide (CO) and hydrocarbons (HC) present in the exhaust gases emitted from the internal engines of automobiles, etc.
As the control for such exhaust gases has become stricter, development of a heater, etc. capable of reducing the emission during the cold start of engine has become more necessary.
In order to respond to such necessity, the present applicant proposed heaters such as mentioned below.
In U.S. Pat. No. 5,318,757 was proposed a honeycomb heater comprising (a) a honeycomb structure and (b) electrodes, wherein slits as a resistance-adjusting means were provided in the honeycomb structure between the electrodes. This heater can quickly heat the low-temperature exhaust gas emitted during engine start.
In U.S. Pat. No. 5,202,547 were proposed a honeycomb heater as shown in FIG. 10, wherein the partition wall "a" at the dead end of each slit 100 (where a current is concentrated most) and the partition walls "b" at the both sides of each slit (where a current is concentrated at a relatively high degree) were made thicker with an electroconductive material; and a honeycomb heater as shown in FIG. 11, wherein the passages "d", "f" and "h" (where a current is concentrated) were sealed with an electroconductive material. In these honeycomb heaters, however, cell rupture occurred at times when heating by electrification was repeated frequently.
In U.S. Pat. No. 5,399,841 was proposed a honeycomb heater wherein the polygonal section of each cell undergoing larger thermal shock and generating a higher thermal stress was formed so as to have a side of 10 mm or less in order to relax the thermal shock and prevent the deformation, cracking, etc. of the cell. However, when this honeycomb heater was actually electrified, cell deformation occurred at the cells of the both sides of each slit end where current concentration took place, and a higher stress appeared at these cells. Therefore, it is necessary to take an appropriate countermeasure to prevent the deformation of such cells and reduce their thermal stress.
In U.S. patent application No. 08/357,213 (European Patent Publication 0 661 097) was proposed a honeycomb heater whose thickness in the direction of gas flow was made smaller at a particular position so as to allow quick local heating. In this heater, however, since the thickness changes sharply from that particular position, current concentration occurred at that position at times during electrification.
As described above, when a honeycomb structure is electrified, there appears (appear) a current-concentrating portion(s). Such portion(s) is (are) in the vicinity of each slit end in the case of a honeycomb structure having a slit(s) and, in the case of a honeycomb structure of vortex type, the center thereof. When the honeycomb structure is subjected to repeated electrification, cell rupture occurs at the current-concentrating portion(s) of the honeycomb structure, which may finally lead to no flow of electricity.